Television receiver



R. 5. HOLMES TELEVISIQN RECEIVER June 18, 1940.

Filed Aug. 20, 1937 2 Sheets-Sheet 1 DEC/(GROUND m6 m w m mm a a M w H m 6 w m a R m R L w m n E B i .7. mu w B0 E m 6 n 5 cr w r MW m s 2 m WW V m 1 n J p. u i t skwkkau ESE June 18,1940.

R. S. HOLMES TELEVISION XEGEIVER Filed Aug. 20. 1937 BRILLIHNCE REFERENCE LEVEL 01" carp -opp THE lI-B-G vSY-S'TEM 2 Sheets-Sheet 2 acne L/cw'r scams DIRK 665MB 1950 v1: LIGHT scams" REDUCED l/v flMPLl'rl/nE ar cou'rnwar CONTROL HEF'BRENCE LE V512 0 GRID VOLTS M WORKING 3/198 THU-S B098 DUB 7'0 fl-B'C-IVOT CHI/{GED IN THE SPECIFIC 6,4817 SHOWN- won/mm B076 DECREJSED Inventor Ralph 6'. 'Holmea (Ittomeg Patent ed June 18, 1940 UNITED s'r-A'rus TELEVISION RECEIVER Ralph 8. Holmes, Haddonfleld, N. J., assignor to Radio Corporation of America, a corporation of Delaware v Application August 20, 1937, Serial No. 160,005

6 Claims. (01. 178-75) scribed in Bedford application Serial No. 728,117;

My invention relates to television receivers and particularly to receivers'employing cathode ray tubes and having means for controlling the picture background.

Specifically, my invention will be described in connection with a receiver having an automatic background control of A. B. C. circuit of the character described and claimed in Poch application Serial No. 718,192, filed March 30, 1934, relatingto Television transmitting and assigned to the Radio Corporation of America... It has been found that the automatic background control action is satisfactory after the initial adjustments have been made providing the "contrast control is not changed at some later time.

The contrast control determines the amount the picture or video signals are amplified and corresponds to the volume contro on a sound broadcast receiver.

If the contrast control is changed, the A. B. C. circuit no longer brings the picture background to the proper value and a manual adjustment of, the picture background is necessary.

It is accordingly an object of my invention to provide an improved television receiver in which an adjustment of the contrast control does not require an additional manual adjustment for obtaining the proper picture background.

A further object of my invention is to provide a television receiver which requires a minimum number of adjustments by the operator when receiving pictures.

In practicing my invention, the television receiver is provided with an automatic background control circuit such as that described in the above-mentioned Poch application. Preferably, a main manual background control device and an auxiliary background control device are pro vided. the. receivers contrast contro being ganged with the auxiliary control device. As will be explained hereinafter, the receiver may be so adjusted that a movement of the contrast control knob will simultaneously change the bias on the cathode ray tube the proper amount to give the correct average illumination or picture background.

The inventionwill be better understood from the following description taken in connection with the accompanying drawings in which,

Figure l is a circuit diagram of a portion of a television receiver embodying my invention,

Figure 2 is a group of curves which illustrate the action of the automatic background control portion of the television receiver, and

Figure 3 is a group of curves which are referred to in explaining the invention.

In Fig. 1 I have shown only that part of a television receiver which is pert nent to the present invention. The receiver is designed for the reception of signals from a transmitter such as that defiled May 29, 1934, entitled Television systems and the method of operation thereof, and as signed to the Radio Corporation of America.

As described in th said Bedford application, horizontal synchroning impulses are transmitted at the end of each scanning line while vertical synchronizing impulses are transmitted at the end of each picture frame. The synchronizing impulses are in the black direction, each impulse consisting of a "pedestal which has on the top thereof a comparatively narrow impulse which is referred to as a super-sync impulse.

' The composite signal consisting of picture signals and synchronizing impulses is illustrated in Fig. 2 where picture signals are indicated at iii, the pedestals at H, and the super-sync impulses at l2. The top of the pedestal usually represents black in the picture although this-is not always the case as the transmitter may be so adjusted that the picture signal representing black does not come to the top of the pedestal. It will be noted that in the drawing only'horizontal synchronizing impulses have been shown.

As explained in the said Bedford application, the composite signal is so transmitted that the height of the pedestals measured from the alter nating current axis of the composite signal is a measure of the picture background, that is, of the average illumination of the scene being transmitted. The super-sync impulses, however, are of fixed amplitude so far as picture background is concerned. This is because the supersync impulses are added to the pedestals after they have been clipped at a level determined by the. picture background.

As will be explained hereinafter, it is because of this fixed amplitude of the super-sync impulses that in previously designed receivers a change in the contrastcontrol threw the background control setting of the receiver out of adjustment.

Referring to Fig. 1, the cathode ray tube l6 of the receiver is supplied with the incoming video signal through video amplifiers ll, l8 and 19 which are preceded by the second detector i of a superheterodyne receiver. The cathode ray tube is of a well-known type comprising a highly evacuated envelope having therein an electron gun consisting of a cathode 2|, control electrode 22 and first anode 23. The tube also has a second anode 24 and a fluorescent screen 26. The cathode ray is electrostatically focused.

The cathode ray is caused to scan the fluorescent screen 26 by means of electromagnetic deflecting coils and their associated deflecting circuits (not shown).

' The video amplifiers l1, l8, and I9 may consist of screen grid tubes which are resistance coupled. In the input circuit of the amplifier l8 suitable contrast control means is provided such as a linear resistor 21 having a movable contact point A thereon.

In the input circuit of amplifier l9, the grid condenser 28 and the grid resistor 29 have such relative values that the synchronizing impulses appearing on the grid amplifier l9 in the positive direction maintain amplifier l9 negatively biased the correct amount of amplifying the picture signals. This negative bias varies in magnitude with variations in the height of the synchronizing impulses. Thus, by connecting the plate of amplifier i3 to the control electrode 22 of the cathode ray tube l6 through a direct current connection as shown, the bias on electrode 22 varies automatically to give the received Dicture the correct average illumination. This automatic background control circuit is described and claimed in the previously mentioned Pooh application.

The complete connection between the output circuit of amplifier I9 and the cathode ray tube electrodes 2| and 22 includes a connection between the cathode 3|;of amplifier tube l9 and the cathode 2 I. This, preferably, includes a space path between cathode 3| and a plate electrode 32 in order to protect the cathode ray tube from injury as described and claimed in Holmes application on Protective devices, Serial No. 713,280, filed February 28, 1934, and assigned to the Radio Corporation of America. The space path 3|-32 is in series with a plurality of resistors 33, 34 and 35, resistors 33 and 34. being shunted by a resistor 36, and the entire series circuit being connected across the plate supply battery 31 of amplifier l8. It will be seen that one end of this series circuit is connected to the negative end of battery 31 through ground and that the other end of the series circuit is connected through a conductor 38 to the positive end of battery 31.

The cathode 2| of the cathode ray tube is connected to a point on the resistor 34 through 2. variable contact point B. Since the resistors 33, 34, 35, 36 and the space path 3|, 32 function as a potentiometer across battery 31, an adjustment of contact point B will adjust the bias applied to the cathode 2|. Thus contact point B is a manual background control device, and will be referred to as the auxiliary control device.

The main control device for manual background control is a variable contact point D on the resistor 35. By moving the control device D, the amount of current flowing through resistors 33, 34 and 35 may be controlled to determine the bias applied to the cathode ray tube control electrode 22.

The position of a contact point C on the resistor 33 determines how great a percentage change in the cathode ray tube bias is produced by a given movement of the contact point B.

In accordance with my invention the contact points A and B are ganged, as indicated by the dotted lines, whereby a movement of the contrast control knob 39 on the front panel of the receiver will cause a movement of contact point B as well as a movement of the contact point A. Thus, if knob 39 is turned to reduce the contrast, the bias on control electrode 22 as determined by contact point B is also reduced and the background illumination of the picture on the screen 26 remains properly adjusted. The operation of the circuit will be better understood by referring to Figs. 2 and 3. The operation of the automatic volume control portion of the circuit is shown in Fig. 2. As explained in the aboveidentified Poch application, a grid leak bias is built up across the resistor 29 by the peaks of the synchronizing impulses causing a flow of grid current. It will be noted that it is the top of the super-sync impulse I2 rather than the top of the pedestal II that determines the bias whereas, as previously explained, the top of the pedestal should control. Also, it will be seen that the synchronizing impulses always come up to a fixed reference level, that is. the point at which the grid of amplifier tube 9 goes positive. As shown in Fig. 2, if the amplitude of the synchronizing impulses becomes less, the bias voltage across resistor 29 decreases until the impulses again drive the grid of *tube l9 positive.

Referring now to Fig. 3, there is shown the action of the circuitat he input electrodes of the cathode ray tube 26 when my invention is employed.

It will be noted that the cathode ray tube has a cut-off point on its characteristic curve, this being the point at which the cathode ray is blocked or cut off. Therefore, this point represents black on the fluorescent screen 26. The bias on the cathode ray tube is so adjusted by control devices B and D that "black in the picture signal just drives the cathode ray tube to cut-oil as indicated. In Fig. 3 the top of the pedestal represents black.

Three different conditions of incoming signals have been illustrated as indicated by the legends at the left of the figure. The curves for the light scene and the dark scene show how the bias is increased automatically in response to a decrease in the height of the pedestals as measured from the A. C. axis whereby the tops of the pedestals drive the cathode ray tube to beam cut-oil? both when there is a light background and super-sync impulses as well as the amplitude of the rest of the signal. In the specific example assumed, theheight of the super-sync impulses from the A. C. axis is substantially the same fo the second and third conditions illustrated. Therefore the bias due to the automatic background control circuit is substantially the same in the two cases.

From an inspection of the curves for the third condition, it will be seen that, if the manual bias were not reduced, the top of the pedestal '(representing black) would drive the cathode ray tube beyond beam cut-off instead of just to beam cutoff. However, if the manual bias is decreased the correct amount at the same time that the contrast is reduced, the black" signal will still just cut ofi the electron beam.

In order to obtain the correct change in cathode ray tube bias for a given change in contrast con- -trol, it is preferred to have the resistor 34 linear knobs on the front panel of the receiver but, preferably, have screw driver adjustments or the like inside the receiver.

The initial adjustment of the receiver is as follows: First the contrast control tap A is moved down to zero for minimum signal, the tap B simultaneously being moved to the most negative end of resistor 34. Then tap D is adjusted until the screen is just black, this being the point at which the cathode ray is just cut off.

Then the contrast control A is moved up to the position of maximum desired signal. Next tap C is adjusted until black in the signal is again black on the screen. This last adjustment is reached when the return line disappears. Preferably resistor 36 has a low resistance compared with the resistance of 33 and 34 in series whereby the adjustment of C does not'necessitate a readjustment of D.

The receiver has now been adjusted so that, at all intermediate settings of the contrast control, black in the scene being transmitted is black on the fluorescent screen.-

I claim as my invention:

1. In a television receiver, a cathode ray tube having input electrodes including a control electrode, an amplifier connected to supply picture signals to said input electrodes, manually operable contrast control means for determining the amount-said signals are amplified in said amplifier, means for varying automatically the bias on said control electrode in response to a change in the average illumination of the transmitted subject, and means operable with said contrast control means for varying the bias on said control electrode in response to a change in the adjustment of said contrast control means.

2. In a television receiver, a cathode ray tube having input electrodes including a control electrode, a second detector for converting a picture modulated carrier wave into video signals, a video amplifier connecting second detector and said input electrodes, manually operable contrast control means for determining the amount said signals are amplified in said amplifier, means for varying automatically the bias on said control electrode in response to a change in the average illumination of the transmitted subject, and means for varying the bias on said control electrode in response to a change in the adjustment of said contrast control means, said last two means varying the bias in a direction to make the average illumination of the reproduced picture correspond to the; average illumination of the transmitted subject.

3. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing impulses, said impulses each consisting of a pedestal which varies in height with respect to the alternating current axis of said composite signal in accordance with the average illumination of the subject being transmitted and a super-sync impulse of fixed amplitude superimposed upon the pedestal, a cathode ray tube having input electrodes including a control electrode, an amplifier connected to supply said composite signal to said input electrodes, manually operable contrast control means for determining the amount said signals are amplified in said amplifier, means for increasing the negative bias on said control electrode in response to a decrease in the height of said pedestals, and

means for decreasing said negative bias in response to said contrast control means being adjusted in a direction to decrease the amplification of said composite signal.

4. In a television receiver, a cathode ray tube having a cathode and a control electrode, an amplifier connected to supply a composite signal comprising picture signals and synchronizing impulses to said cathodeand said electrode, adjustable means for varying the amount said signals and said impulses are amplified in said amplifier, means for biasing said control electrode, means for so varying the bias on said control electrode in response to a change in the height of said impulses with respect to the alternating current axis of said composite signal that the background of the reproduced picture is at least approximately the same as the gackground of the reproduced view, additional adjustable means for varying the bias on said control electrode, and means for so ganging said two adjustable means that said bias is decreased in response to an adjustment of said first adjustable means in the direction to reduce the amplification of said composite signal.

5. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing impulses, said impulses consisting of a pedestal which varies in height with respect to the alternating current axis of said composite signal in accordance with the average illumination of the view being transmitted and a super-sync impulse which does not vary in amplitude in accordance with said illumination, a cathode ray tube having input electrodes including'a control electrode, means for biasing said electrode, an amplifier connected to supply said composite signal to said input electrodes, manually adjustable means for varying the amount said composite signal is amplified in said amplifier, means for varying the bias on said control electrode in response to a change in the height of said synchronizing impulses with respect to said alternating current axis, adjustable means for varying the bias on said control electrode, and means for so gauging said two adjustable means that said bias is decreased in response to an adjustment of said manually adjustable means in the direction to reduce the amplification of said composite signal.

6. In a television receiver for the reception of a composite signal comprising picture signals and synchronizing impulses, said impulses consisting of a pedestal which varies in height with respect to the alternating current axis of said composite signal in accordance with the average illumination of the view being transmitted and a super-sync impulse which does not vary in amplitude in accordance with said illumination, a cathode ray tube having input electrodes including a control electrode, means for biasing said control electrode, an amplifier connected to supply said composite signal to said input electrodes, manually adjustable contrast control means for varying the amount said composite signal is amplified insaid amplifier, and means for maintaining the bias on said control electrode such that regardless of the setting of said contrast control black in the scene being transmitted is black on the fluorescent screen of said cathode ray tube.

RALPH S. HOLMES. 

